APPLICATION NOTE

Correlative Confocal
Raman Microscopy for
2D Materials Investigation

APPLICATION NOTEThe Raman principle

No need for compromises

The Raman effect is based on theinelastic scattering of light by themolecules of gaseous, liquid or solidmaterials. The interaction of a moleculewith photons causes vibrations of its
chemical bonds, leading to specificenergy shifts in the scattered light. Thus,
any given chemical compound produces
a particular Raman spectrum when
excited and can be easily identified bythis individual “fingerprint.”

The Raman effect is extremely
weak, so every Raman photon isimportant for imaging. Therefore
WITec Raman imaging systems
combine an exceptionally sensitive
confocal microscope with an ultra-highthroughput spectrometer (UHTS).
Precise adjustment of all optical andmechanical elements guarantees
the highest resolution, outstanding
speed and extraordinary sensitivity –simultaneously!

Raman spectroscopy is a wellestablished, label-free and nondestructive method for analyzing themolecular composition of a sample.

Raman imaging
In Raman imaging, a confocal
microscope is combined with aspectrometer and a Raman spectrum
is recorded at every image pixel. The
resulting Raman image visualizes thedistribution of the sample’s compounds.
Due to the high confocality of WITec
Raman systems, volume scans and 3D
images can also be generated.

This optimization allows the detectionof Raman signals of even weak Raman
scatterers and extremely low material
concentrations or volumes with thelowest excitation energy levels. This isan unrivaled advantage of WITecsystems.

Resolution
Lateral resolution is
physically limited to~200 nm, dependingon the wavelength of
the incident light.

Speed
The more sensitive a system is, theshorter the acquisition time for a singlespectrum. WITec’s Ultrafast Raman
Imaging reduces acquisition times forsingle Raman spectra down to well
below 1 ms.

Sensitivity
A high confocality increases the signal-to-noise ratio by
reducing the background. With the UHTS Series, WITecdeveloped lens-based, wavelength-optimized spectrometerswith a spectral resolution down to 0.1 cm-1 relativewavenumbers.

WITec Wissenschaftliche Instrumente und Technologie GmbH
Lise-Meitner-Str. 6, D-89081 Ulm, Germany
Tel. +49 (0) 731 140 700, Fax +49 (0) 731 140 70200
info@WITec.de, https://raman.oxinst.com

Analyses of 2D materials with correlative Raman microscopy
The mechanical isolation of single-layer
graphene with the help of adhesive tapein 2004 kicked off the rapid growth inresearch on materials consisting of only
one or a few layers of atoms, defined as
2D materials. Different from bulk matter,they display interesting semiconducting
characteristics that inspired an extensive
exploration of their structural, electronicand optical properties. While graphene
is still the most prominent example of asingle-layered semiconducting material,other 2D materials such as transition
metal dichalcogenides (TMDCs) andblack phosphorus also possess sizeable

bandgaps. In this application note wewill introduce both confocal Raman
microscopy as a stand-alone techniqueand correlative Raman microscopy
for the comprehensive investigation ofvarious 2D materials.
Confocal Raman imaging is an ideal
method for studying 2D thin films, revealing
their molecular characteristics through a
non-destructive and fast procedure. It is
used to discern the orientation of theirlayers and to investigate defects, strain
and functionalization, as their Raman
properties are determined by molecularbonds, relative orientation and numberof layers. Their morphological details canbe visualized with other high-resolution
microscopy methods such as ScanningElectron Microscopy (SEM). By correlating
the information of both approaches, 2Dmaterials can be even more thoroughly
analyzed.
WITec has developed microscopes that
integrate confocal Raman imaging andoptionally Atomic Force Microscopy(AFM), Scanning Near-field OpticalMicroscopy (SNOM) or SEM into one
instrument for highly sensitive andefficient correlative microscopy.

Correlative topography and Raman measurements

In Figure 1 the SEM (A) and RISE (C)images of a CVD-grown graphene sheet
deposited on a Si/SiO2 substrate arepresented. Layer numbers were deduced
from the Raman spectra (Figure 1B).
The colors used represent a single layer(blue, dark green) with different grade ofdefects and more than one layer (lightgreen, red). The RISE image highlights
the large amount of defects in the blue
area, in good agreement with the highresolution SEM image.

Figure 1: RISE microscopy of graphene.
(A) SEM image of a graphene sample. (B)Raman spectra of graphene. (C) SEM image
overlaid with the color-coded confocal
Raman image. The colors display thegraphene layers and wrinkles.
Image parameters: 20 x 20 µm2, 150 x 150
pixels = 22,500 spectra, integration time: 0.05
s/spectrum, 1 layer (blue, dark green), morethan 1 layer (light green, red), Si (yellow).

AB

Intensity [a.u.]

RISE Microscopy is the combination ofconfocal Raman Imaging and ScanningElectron Microscopy (SEM). It incorporates
the sensitivity of the non-destructive,
spectroscopic Raman technique alongwith the high-resolution of electron
microscopy.

relative wavenumbers [cm-1]

CAPPLICATION NOTERISE microscopy of MoS2
Here the green spectrum indicatesmonolayer MoS2, while the blue andred spectra signify edges and defects,
respectively.

CVD-grown monolayers of MoS2 form
triangular two-dimensional crystals.
Twin crystals of MoS2 on SiO2/Si appear
in the SEM image as star-shaped forms(Figure 2B). The Raman spectra of MoS2monolayers
show the characteristicE‘/E2g and A‘1/A1g Raman band modesof MoS2 at 385 and 403 relativewavenumbers (cm-1), respectively (Figure2A). With an increasing number of layers
the two Raman bands drift apart due toin-plane and out-of-plane (inter-layer)
vibrations.

The spectral information was used
to create a Raman image which wasoverlaid on the SEM image to resultin a RISE image (Figure 2C). The grainboundaries visible in the SEM image
correlate perfectly with the areas where
Raman signals indicate crumpled
or overlapping edges, highlighting
the effectiveness of RISE imaging forcharacterizing MoS2 crystals.

BA

Intensity [a.u.]

RISE microscopy reveals structure
as well as crystalline and exciton
dynamics of few-layered transition metal
dichalcogenides (TMDCs).

relative wavenumbers [cm-1]

CFigure 2: RISE microscopy of MoS2 crystals.
(A) Raman spectra of MoS2: green = 1L MoS2, blue and red = edges and defects. (B) SEM image of MoS2 crystals. (C) Color-coded Raman image
derived from the spectral information overlaid on the SEM image to produce the RISE image.
Sample courtesy of Ting Yu, Nanyang Technological University, Singapore.

WITec Wissenschaftliche Instrumente und Technologie GmbH
Lise-Meitner-Str. 6, D-89081 Ulm, Germany
Tel. +49 (0) 731 140 700, Fax +49 (0) 731 140 70200
info@WITec.de, https://raman.oxinst.com

Correlative imaging reveals layers and defects of WS2 crystals

The number of layers and the type oflayer stacking influences not only Ramanpeaks’ intensities and positions but also
the intensity of photoluminescence peaks
and their positions. Due to its direct bandgap, monolayer WS2 shows a strong PL
around 635 nm when excited with a 532
nm laser (Figure 3C). This PL decreases
significantlywithincreasinglayer
numbers because the direct bandgap
that is present solely in single-layered
TMDCs allows for direct recombination ofelectrons and holes (Figure 3D). Also thePL peak position varies and PL lifetimechanges with thickness: the more layers
the more PL shifts to the red (Figure 3E)
and PL lifetime grows shorter (Figure 3F).

ABCD

Intensity [a.u.]

WS2 crystals CVD-grown on a Si/SiO2
substrate form triangular islands. AtomicForce Microscopy (AFM) was used tomeasure their layer thickness (Figure 3A).
The flakes are comprised of 1, 2 andmore layers. The Raman spectrum of
WS2 displays a typical in-plane (E’/Eg)
mode at 352 relative wavenumbers(cm-­1) and an out-of-plane (A’1/A1g) mode
at 416 relative wavenumbers (cm-1)[1]. The Raman image of the intensity at
416 relative wavenumbers (cm-1) showsthat this signal is strongest in those partsof the crystals that consist of two or morelayers and defects, respectively (Figure3B).

relative wavenumbers [cm-1]

EFGH

Scanning Near-field Optical Microscopy(SNOM) with up to 60 nm spatial
resolution was also used for TMDC
characterization [2,3]. Here, strong PL
indicates edge defects in the material,
as shown in a high-resolution SNOM
image of another WS2 crystal (Figure 3G).
These can also be deduced from PL peak
position changes (Figure 3H).
Figure 3: Correlative AFM-SNOM-PL imaging of WS2.
(A) AFM image of CVD-grown WS2 flakes on a Si/SiO2 substrate with varying numbers of layers(1, 2, >2). (B) A peak intensity map of the 416 relative wavenumbers (cm-1) Raman mode. (C)Upon excitation with wavelength of 532 nm, WS2 shows a very strong PL around 635 nm.
(D) PL intensity map of the WS2 flakes at 635 nm corresponds exactly to the thickness of thematerial. (E) Position map of PL maxima. (F) PL lifetime map. (G) A high-resolution SNOM-PL
image of a WS2 flake reveals edge defects that correlate with strong PL. (H) The edge defects
are also visible in the high-resolution SNOM-PL peak position image.
Sample courtesy of Ting Yu, Nanyang Technological University, Singapore.

APPLICATION NOTERISE imaging and in situ modification of WS2
Here we present the analysis and RISE
imaging of a multi-layered WS2 crystal. Itshows the typical triangular shape of thematerial, as well as the heterogeneity ofthe surface.
A strong WS2 Raman signal wasreported at approximately 350 relative
wavenumbers (cm-1) [4]. Researchers
were able to divide this peak into three
sub-peaks [1]. They associated a signal
at 344 relative wavenumbers (cm-1) with
the in-plane E‘ Raman mode. Using thisspectral band we generated a Raman
intensity map. Overlaying it with the SEM
data produced a RISE image (Figure 4B).
The Raman signals show that this flake
is not homogenously layered. It contains
1L layers and areas with more layers
and also defects. The Raman imagecorresponds very well to the structures
revealed by SEM.
Figure 4C shows the overlay of the SEM
image and an intensity peak map of PL.
The WS2 crystal displays strong andhomogenous PL at about 640 nm only inthe 1L areas due to the direct bandgap inmonolayer WS2. Defects seen in the SEM
image exhibit PL intensity variations rich indetail. PL intensity perfectly corresponds
to the intensity of the Raman signal.

ABCFigure 4: Correlative RISE and SEM-PL microscopy of WS2 crystals.
(A) The SEM image of a WS2 flake reveals defects on the surface and probably adsorbed molecules. (B) The correlative RISE image is composed of the SEM image and the corresponding
Raman intensity map over the 344 cm-1 Raman band. The Raman image identifies 1L (cyan),
2L (blue) and 3L (red) areas. (C) Overlay of the SEM and PL intensity images shows that the 3L
area lacks PL (upper part of the triangle). Excitation wavelength for Raman and PL imaging =532 nm.
Sample courtesy of Ting Yu, Nanyang Technological University, Singapore.

WITec Wissenschaftliche Instrumente und Technologie GmbH
Lise-Meitner-Str. 6, D-89081 Ulm, Germany
Tel. +49 (0) 731 140 700, Fax +49 (0) 731 140 70200
info@WITec.de, https://raman.oxinst.com

Low-frequency Raman modes and imaging of MoSe2
Not every transition metal dichalcogenide(TMDC) can be adequately characterized
by its vibrational modes between 200
and 500 relative wavenumbers. In somematerials typical in-plane (E‘/E2g) andout-of-plane (A‘/A‘1g) modes, thoughchanging in intensity with increasing
thickness of the material, do not show
peak shifts. Also, these primary Ramanpeaks are highly sensitive to strain andfolding [5] and therefore cannot be used
to unambiguously determine the numberof layers.

numbers between 10 and 50 cm-1,efficient blocking of the Rayleigh line
while still transmitting adjacent light isnecessary. WITec has developed theRayShield coupler with exactly thesefeatures that can be combined withevery WITec Raman microscope and isavailable for 488, 532, 633 and 785 nm
lasers.

Intensity [a.u.]

All measurements of Figure 5 werecarried out by Maria O‘Brien, Nigel
McEvoy and colleagues (Trinity College,Dublin). The low-frequency spectrum
However, low-frequency in-plane shear shows that 1L MoSe2 has no Raman
modes (SM) and out-of-plane layer-​ peak in this range (Figure 5A). With
breathing modes (LBM) modes are increasing layer numbers SM andmuch more revealing [6]. To measure LBM peaks occur, shifting in positions
Raman spectra close to the laser’s and intensities. Accordingly, the flakesexcitation wavelength at low wave- that appear to be the thinnest in theoptical image (Figure 5B) are almost
invisible in the Raman image, whereasA
thicker material can be detectedby their Raman modes (Figure 5C).
The type of layer stacking also influences
the Raman peaks’ intensities andpositions. In stable, semiconducting
MoSe2
withtrigonal
prismatic
coordination the individual layers cancombine in two arrangements, called
relative wavenumbers [cm-1]

BC

H and R stacking. These so-called
polytypes cannot be distinguished fromeach other in optical images but canbe by their distinctive Raman peaks.
Intensity changes in Raman modes at
19 and 24 relative wavenumbers (cm-1)
reflect differences in the layer stacking.
2L MoSe2 displays a typical Raman
signal at 19 relative wavenumbers (cm-1)
that is more intense in the 2H than in the3R stacking configuration (Figure 5D).
From the peak maximum map at 24
relative wavenumbers (cm-1) it is clearthat 3L MoSe2 shows the highest intensity
in the 3R3R configuration followed by
2H3R and inverted 3R3R stacking (Figure5E). O‘Brien and colleagues concluded
that „... low-frequency Raman mapping
can distinguish between different
stacking configurations rapidly and nondestructively, allowing TMDCs in different
stacking configurations to be identified
and studied without the need for highresolution imaging.“References[1] Peimyoo N., et al. (2013). Nonblinking,intense two-dimensional light emitter:monolayer WS2 triangles. ACS nano,7(12), 10985-10994.
[2] Lee, Y., et al. (2015). Characterizationof the structural defects in CVD-grownmonolayered MoS 2 using near-field
photoluminescence imaging. Nanoscale,7(28), 11909-11914.

DE[3] Lee, Y., et al. (2017). Near-field
spectral mapping of individual
exciton complexes of monolayer WS2
correlated with local defects and charge
population. Nanoscale, 9(6), 2272-2278.
[4] Wang, Y., et.al. (2015). Strain-induced
direct–indirect bandgap transition andphonon modulation in monolayer WS2.
Nano Research, 8, 2562-2572.

Figure 5: Low-frequency Raman modes of MoSe2.
(A) Low-frequency Raman spectra of 1L, 2L and 3L MoSe2 with various layer configurations (Hand R). (B) Optical image of CVD-grown MoSe2 reveals differences in thickness and structure ofthe crystals. (C) Map of position of maximum peak intenstiy over the range of 10 to 40 relativewavenumbers (cm-1). (D) Peak intensity map of 2L MoSe2 at 19 relative wavenumbers (cm-1).
(E) Peak intensity map of 3L MoSe2 at 24 relative wavenumbers (cm-1).
Images courtesy of O‘Brien, McEvoy and colleagues, Trinity College, Dublin, Ireland.

[5] Zhang, X., et al. (2013). Raman
spectroscopy of shear and layer
breathing modes in multilayer MoS2.
Physical Review B—Condensed Matter andMaterials Physics, 87(11), 115413.
[6] O’Brien, M., et al. (2016). Mapping of
low-frequency Raman modes in CVDgrown transition metal dichalcogenides:
Layer number, stacking orientation andresonant effects. Scientific reports, 6(1),19476.

WITec Microscopesalpha300 S:
Scanning Near-field
Optical Microscopealpha300 A:
Atomic ForceMicroscopealpha300 R:
Confocal Raman
Microscopealpha300 apyron™: AutomatedConfocal Raman Microscope

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RISE®: Raman Imaging
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